I hate to be the one to break this to you, but most likely she has another man. The cutting off from family, wearing younger clothes, going out with single friends (or maybe just ONE single guy friend I'm guessing) sure look likely. If you could access your cell phone records I bet you'd see numerous calls to a particular number.
You don't mention whether your tumor was benign or malignant. Some people cannot handle their partner having a life-threatening disease, and they bail out early rather than go through the pain of losing the partner.
Brain surgery, even for a benign tumor, can also sometimes result in personality changes. You might ask other friends or family if they have noticed anything different about you. But my guess is that she feels overwhelmed by motherhood, scared by your illness, and turned to another man for excitement. It's relatively rare for a woman with such a young child to bail like this in the absence of abuse or her having an affair.
